1. Field of the Invention
This invention relates to an automatic focusing system, and particularly to a counter-measure when focus detection is difficult or impossible.
2. Description of the Prior Art
The automatic focusing system, irrespective of the TTL (Through The Lens) type or the non-TTL type, has a focus detection capable range of predetermined magnitude. That is, for example, in the case of a camera, the picture-taking lens thereof generally has a focus adjustable range of the order of 0.5 m or 1 m to the order of 10 m or 15 m (such object distances generally correspond to "infinity"), but in contrast, the ordinary focus detecting system can only take, as the object of focus detection, for example, several meters before and after 5 m to which the picture-taking lens is focused, and accordingly, where the object to be photographed lies outside the focus detection capable range, the focus detection with respect to such object is very difficult or entirely impossible.
This tendency is salient particularly in long-focus lenses or macro lenses and, above all, in single lens reflex cameras using the TTL type system, it offers a great problem. That is, for example, in single lens reflex cameras, more severe focusing accuracy is required while, on the other hand, where the focus detecting system is of a so-called image sharpness detection type or contrast detection type, the magnitude of the focus detection capable range becomes smaller and smaller and accordingly, the problem as noted above is feared more strongly.